A liquid crystal polyester is a polymer with rigid molecular chains. It shows high strength and high elastic modulus among fibers, which are produced from a melt-spinning process, since solid-phase polymerization is performed with the molecular chains highly-oriented in a fiber axial direction during the melt-spinning process. As shown in Non-patent document 1, the liquid crystal polyester has thermal resistance and dimensional stability since the solid-phase polymerization increases its molecular weight to raise its melting point. Thus, the liquid crystal polyester fibers have high-strength, high elastic modulus, excellent thermal resistance and excellent thermal dimensional stability by applying the solid-phase polymerization. On the other hand, the liquid crystal polyester fibers have disadvantage, such that the interaction perpendicular to fiber axial direction is low and that abrasion resistance is so inferior that fibrillation may be caused by frictions, because rigid molecular chains are highly oriented in a fiber axial direction to form dense crystals.
Recently, specifically for filters and screen-printing gauzes which are made of monofilaments, higher weaving density (making a mesh higher) and larger opening section areas are demanded in order to improve the performance. To achieve this, the higher single-fiber fineness, the higher strength and the higher elastic modulus are strongly demanded. The liquid crystal polyester fibers are being counted on because of its high strength and high elastic modulus. At the same time, they require to reduce defects at the opening sections to improve performance. However, the liquid crystal polyester fibers have disadvantage against reducing defects at the opening sections, because they do not have enough abrasion resistance to prevent from fibrillation. The fibril is generated by friction during a fiber manufacturing process, and fibers are required to be improved in its abrasion resistance. In addition, the fibril may be also generated from fusion-bonding in the solid-phase polymerization. The fusion-bonding may cause a defect to reduce the fiber lengthwise strength and the uniformity of the fineness. Therefore the fusion-bonding defect is required to be reduced, which means the fiber lengthwise strength or the uniformity of the fineness are required to be improved. Process passability in a higher-order fabric processing such as a weaving process, and weavability itself are worsen, because the fibrils are caught or deposited onto the guide to cause a tension fluctuation. Therefore the liquid crystal polyester fibers are required to improve in the uniformity of fiber-lengthwise strength and fineness and the abrasion resistance.
In order to improve an abrasion resistance of liquid crystal polyester fibers, Patent documents 1-3 disclose a technology in which liquid crystal polyester fibers are heated at (Tm1)+10° C., where an endothermic peak temperature (Tm1) is observed by differential calorimetry under a temperature elevation of 20° C./minute from 50° C. Though that technology can improve the abrasion resistance well, it cannot sufficiently improve the uniformity in the fiber lengthwise direction. The technology disclosed in Patent documents 1-3 comprises two processes of which one is a process where liquid crystal polyester fibers, which have been solid-phase polymerized in a fibrous state, are unrolled and once taken up, and of which the other is a process where fibers are again unrolled from the taken-up packages and subjected to high-temperature heat treatment. Such unrolling process after solid-state polymerization has to be separated from the high-temperature heat treatment process, because the speed in the unrolling process after the solid-state polymerization cannot be easily kept constant and therefore the unrolling process cannot be continued to the high-temperature heat treatment process where the speed is constant. The reason why the speed in the unrolling process after the solid-state polymerization cannot be easily kept constant is explained as follows. Because a package after the solid-state polymerization has been slightly fused, if the fusion-bonding part is strongly forced to be exfoliated, fibers will be fibrillated. In case of such a trouble, it is desirable for the package to be positively driven to rotate so as to feed fibers by unrolling with lower tension. In addition, if the rotation speed is constant the feeding speed gradually decreases as the package is reduced in winding amount. Besides, the feeding speed can be kept constant by controlling the rotation speed. However, such a control method is difficult to be applied to certain fibers like liquid crystal polyester which has low elongation and high elastic modulus, and therefore the tension fluctuates to cause yarn breakage as the speed (rotation speed) changes in a short period.
The technologies in Patent documents 1-3 have problem that fibers polymerized in solid phase are unrolled and once taken up. In order to take the fibers up without loosening, they have to be tensed and taken up into a package as being traversed with a yarn path guide. However, if a yarn, having low abrasion resistance, which has been polymerized in solid phase, is tensed and traversed with the guide, the fibers are slightly fibrillated because both ends of the traverse are specifically tensed. Such a slight fibrillation does not influence the fineness and the strength of fibers so much. However, when exfoliated fibrils are deposited on the traverse guide, the tension fluctuation increases, as the winding amount increases with time passage. That may cause a yarn breakage derived from sudden tension malfunction and also cause a great fibrillation to locally lower the fineness and the strength. Further, the deposited fibril may be rolled up in fibers, causing a trouble that the fineness locally increases. Furthermore, if such a solid-phase polymerized yarn having local abnormal fineness is heat treated at a high temperature, the generation of fibrils worsens the running stability and the tension fluctuation increases at the high-temperature heat treatment as causing the fineness malfunction further. In addition, meltdown of fibers may be caused.
Although an oil addition is desirable for preventing from the fusion-bonding in solid-phase polymerization, the oil deposits as a sublimate in a high-temperature heat treatment equipment so as to contact with fibers running over time and then cause a trouble of tension fluctuation because the high-temperature heat treatment is performed after the solid-phase polymerization at a temperature higher than the temperature of the solid-phase polymerization. When the deposit is rolled up and remains on final products, it may deposit on a weaving machine guide, etc., as causing troubles of defective running or yarn breakage. Thus oil for the solid-phase polymerization should be washed away during unrolling after solid-phase polymerization, however, the washing may reduce the oil function of reducing frictional resistance. That may cause the running tension rising and fibrillation, and therefore take-up of the solid-phase polymerized yarn may be further disadvantageous.
Thus, though the technologies, which are disclosed in Patent documents 1-3, do not tend to expose a problem in a case of several tens of thousands meters, local malfunctions may occur in a longitudinal direction of fibers if continuous throughput increases. That occurs for the reason that fibers after the solid-phase polymerization are unrolled and are once taken up.
Patent documents 1 and 3 refer to the high-temperature heat treatment by a phrase “it can be continued as unrolling fibers from the package”, and they refer to the unrolling by a phrase “the package, which has been polymerized in solid phase, is preferably rotated by active driving”. The running speed is not constant if the package, which has been polymerized in solid phase, is rotated as described above. Therefore in such a condition, when the unrolling and the high-temperature heat treatment are performed successively the heat treatment speed does not become constant so as to generate unevenness of strength and abrasion resistance in a lengthwise direction. In addition, because fluctuation of tension for unrolling affects the high-temperature heat treatment process directly, so that the tension fluctuates and troubles, such as yarn breakage and fineness malfunction, are caused.
Patent document 1 discloses neither a washing of oil which is polymerized in solid phase nor a successive unrolling-heat treatment of an oil-free yarn which is polymerized in solid phase. Though Patent document 3 discloses the washing, it does not disclose the successive unrolling-heat treatment as a technical suggestion. In addition, in processes disclosed in Patent documents 1-3, treated fibers are taken up in each step, so that the rise of cost, such as the take-up equipment cost and the labor cost for driving workers, has been unavoidable.
As described above, conventional technologies have not suggested a method for producing liquid crystal polyester fibers, which have a high strength, a high elastic modulus, a high thermal resistance and a high abrasion resistance, without fibril defects in low cost and good yield.